Methods and apparatuses for using assistance data relating to satellite position systems

ABSTRACT

Methods and apparatuses to obtain an ordered set of satellite positioning system (SPS) satellites, in view of a mobile SPS receiver, via one or two-way communication with the mobile SPS receiver. The mobile SPS receiver receives, in one embodiment, an ordered set of SPS satellites from a cellular transmission site. The ordered set of satellites are those in view of the mobile SPS receiver at a given time; such that the mobile SPS receiver may search for the SPS satellites according to an order of the ordered set of SPS satellites. The order of the ordered set may be obtained by various methods one of which is by minimizing the geometric dilution of precision (GDOP) Satellite; health data may be included in the transmission.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/813,597, filed on Mar. 20, 2001 now U.S. Pat. No. 6,720,915, and isrelated to and hereby claims the benefit of the filing dates of twoprovisional applications by the same inventor, Leonid Sheynblat. Thefirst provisional application is entitled, Methods and Apparatus forUsing Assistance data Relating to Satellite Position Systems, Ser. No.60/190,600, filed on Mar. 20, 2000. The second provisional applicationis entitled Method and Apparatus for Using Satellite Status Informationin Satellite Positioning Systems, Ser. No. 60/228,258, filed on Aug. 25,2000.

FIELD OF THE INVENTION

The present invention relates to receivers capable of determiningposition information of satellites and in particular, relates to suchreceivers which find application in satellite positioning systems (SPS)such as the U.S. Global Positioning System (GPS).

BACKGROUND OF THE INVENTION

GPS receivers normally determine their position by computing times ofarrival of signals transmitted simultaneously from a multiplicity of GPS(or NAVSTAR) satellites. These satellites transmit, as part of theirmessage, both satellite positioning data as well as data on clocktiming, so-called “ephemeris” data. The process of searching for andacquiring GPS signals, reading the ephemeris data for a multiplicity ofsatellites and computing the location of the receiver from this data istime consuming, often requiring several minutes. In many cases, thislengthy processing time is unacceptable and, furthermore, greatly limitsbattery life in miniaturized portable applications.

GPS receiving systems have two principal functions. The first is thecomputation of the pseudoranges to the various GPS satellites, and thesecond is the computation of the position of the receiver using thesepseudoranges and satellite timing and ephemeris data. The pseudorangesare simply the times of arrival of satellite signals measured by a localclock. This definition of pseudorange is sometimes also called codephase. The satellite ephemeris and timing data is extracted from the GPSsignal once it is acquired and tracked. As stated above, collecting thisinformation normally takes a relatively long time (30 seconds to severalminutes) and must be accomplished with a good received signal level inorder to achieve low error rates.

Most GPS receivers utilize correlation methods to compute pseudoranges.These correlation methods are performed in real time, often withhardware correlators. GPS signals contain high rate repetitive signalscalled pseudorandom (PN) sequences. The codes available for civilianapplications are called C/A (coarse/acquisition) codes, and have abinary phase-reversal rate, or “chipping” rate, of 1.023 MHz and arepetition period of 1023 chips for a code period of 1 millisecond. Thecode sequences belong to a family known as Gold codes, and each GPSsatellite broadcasts a signal with a unique Gold code.

For a signal received from a given GPS satellite, following adownconversion process to baseband, a correlation receiver multipliesthe received signal by a stored replica of the appropriate Gold codecontained within its local memory, and then integrates, or low-passfilters, the product in order to obtain an indication of the presence ofthe signal. This process is termed a “correlation” operation. Bysequentially adjusting the relative timing of this stored replicarelative to the received signal, and observing the correlation output,the receiver can determine the time delay between the received signaland a local clock. The initial determination of the presence of such anoutput is termed “acquisition.” Once acquisition occurs, the processenters the “tracking” phase in which the timing of the local referenceis adjusted in small amounts in order to maintain a high correlationoutput. The correlation output during the tracking phase may be viewedas the GPS signal with the pseudorandom code removed, or, in commonterminology, “despread.” This signal is narrow band, with a bandwidthcommensurate with a 50 bit per second binary phase shift keyed (BPSK)data signal which is superimposed on the GPS waveform.

The correlation acquisition process is very time consuming, especiallyif received signals are weak. To improve acquisition time, most GPSreceivers utilize a multiplicity of correlators (up to 36 typically)which allows a parallel search for correlation peaks.

Conventional GPS receiving equipment is typically designed to receiveGPS signals in open spaces since the satellite signals are line-of-sightand can thus be blocked by metal and other materials. Improved GPSreceivers provide signal sensitivity that allows tracking GPS satellitesignals indoors, or in the presence of weak multipath signals or signalsthat are pure reflections. The ability to acquire such weak GPS signals,however, typically causes other problems. For example, the simultaneoustracking of strong and weak signals may cause the receiver to lock on toa cross-correlated signal, which is not a true signal. Instead offinding a weak true peak, a stronger cross-correlated peak may beacquired. Tracking a weak satellite signal does not guarantee that it isa direct signal. This weak signal may be a reflected signal or acombination of direct and indirect signals. The combined signals arereferred to as multipath signals. The path of the reflected signal istypically longer than the path of the direct signal. This difference inpath length causes the time-of-arrival measurement of the reflectedsignal to be typically delayed or the corresponding code phasemeasurement to contain a positive bias. In general, the magnitude of thebias is proportional to the relative delay between the reflected anddirect paths. The possible absence of a direct signal component makesthe existing multipath mitigation techniques (such as a narrowcorrelator or a strobe correlator) obsolete.

The GPS navigation message is the information transmitted to a GPSreceiver from a GPS satellite. It is in the form of the 50 bit persecond data stream that is modulated on the GPS signals.

The data message is contained in a data frame that is 1500 bits long. Ithas five subframes each of which contains GPS system time. Each subframeconsists of 10 words of 30 bits each. Subframes 1 through 3 are repeatedevery 30 seconds. There are twenty-five pages of data appearing insequence in the fourth and fifth subframes; one every 30 seconds. Thus,each of these twenty-five pages repeats every 750 seconds.

Subframes 4 and 5 contain two types of health or status data for the GPSsatellites: (a) each of the 32 pages which contain the clock/ephemerisrelated almanac data provide an eight-bit satellite health status workregarding the satellite whose almanac data they carry, and (b) the25^(th) page of subframe 4 and 5 jointly contain six-bit health statusdata for up to 32 satellites. Additional satellite health data are givenin subframe 1.

Typically, a GPS receiver will receive information concerning the status(e.g. “health”) of a satellite and then process the GPS signals by notacquiring and not tracking unhealthy satellites while it acquires andtracks GPS signals from healthy satellites. Alternatively, standaloneGPS receivers can be designed to acquire and track unhealthy satellitesbut avoid using their signals in the location computation after havingread the health status data from the ephemeris message from an unhealthysatellite's signal (see the related provisional patent applicationMethod and Apparatus for Using Satellite Status Information in SatellitePositioning Systems, Serial No. 60/228,258, filed on Aug. 25, 2000,which is hereby incorporated herein by reference.)

Satellite position systems have used various types of assistance data toimprove the performance of an SPS receiver. For example, an SPS receivermay receive Doppler estimates from an external source (e.g. a radiotransmission to the SPS receiver). Another type of assistance data maybe the identification of satellites in view of the estimated or knownlocation of the SPS receiver. In the past, the identification of thesesatellites has not included any indication of whether the satellites mayhave a poor geometry relative to the estimated location of the SPSreceiver or relative to each other. Also, in the past, theidentification of satellites in view of an SPS receiver has not includedan indication of poor geometry with satellite health data.

SUMMARY OF THE INVENTION

The present invention relates to methods and apparatus for determiningan ordered set of SPS satellites in view of a mobile SPS receiver. Onemethod includes determining an ordered set of SPS satellites in view ofa position (e.g. a representative position) in a cell of a cellularcommunication system and then transmitting the ordered set of SPSsatellites from a cellular transmission site located within or near thecell such that a SPS receiver located within the cell of the cellularcommunication system may receive the ordered set of SPS satellites.

The ordering of SPS satellites in the ordered set may be performedaccording to different methods, such as by minimizing a geometricdilution of precision (GDOP); by minimizing a position dilution ofprecision (PDOP), by minimizing a horizontal dilution of precision(HDOP), by providing a position solution which uses SPS satelliteshaving a desired geometry relative to one another, by providing aposition solution which uses SPS satellites having a desired geometryrelative to the mobile SPS receiver; ordering based upon a probabilityof SPS satellite signal acquisition; ordering based on an estimate ofmeasurement quality from the ordered set of SPS satellites; orderingperformed by providing an optimal geometric trilateration solution, andordering based on user defined selection criteria. Additionally, theordering may include satellite health information.

An apparatus, in one embodiment, for creating an ordered set of SPSsatellites includes; a server to determine an ordered set of SPSsatellites in view of a cell of a cellular communication system at agiven time and a transmitter, coupled to the server, to transmit theordered set of SPS satellites from a cellular transmission site locatedwithin or near the cell. Thus, a mobile SPS receiver located within thecell may receive the ordered set of SPS satellites.

The server, in one embodiment, further includes a processor; and aninformation source coupled to the processor. The information sourcecontains sets of SPS satellites in view of cells of the cellular servicearea and the processor determines the ordered set of SPS satellites forthe cell within the cellular service area. The server may be a GPSreference server, a cellular switching center, a location server, acellular transmission site, a base station controller or a mobile SPSreceiver.

A method, in one embodiment, for obtaining an ordered set of SPSsatellites, in view of a mobile SPS receiver includes receiving anordered set of SPS satellites via a cellular transmission from acellular transmission site, by a mobile SPS receiver configured toreceive both SPS signals and signals transmitted from the cellulartransmission site. Thus, allowing the mobile SPS receiver to search forthe SPS satellites according to an order of the ordered set of SPSsatellites obtained from the transmission. The mobile SPS receiver maymodify searching for SPS satellites before or after acquisition of theSPS satellites based on SPS satellite health data.

An apparatus, in one embodiment, for receiving an ordered set of SPSsatellites includes a mobile SPS receiver to receive SPS signals; and areceiver configured to receive signals transmitted from a cellulartransmission site; such that an ordered set of SPS satellites may betransmitted via the cellular transmission site to the receiver and themobile SPS receiver may search for the SPS satellites according to anorder of the ordered set of SPS satellites.

Another embodiment, of the present invention, provides methods andapparatus enabling two-way communication with a mobile SPS receiver. Amethod according to this embodiment, of the present invention, includesreceiving a transmission from a mobile SPS receiver within a cell of acellular service area, the mobile SPS receiver being configured totransmit and receive cellular signals; determining an ordered set of SPSsatellites in view of the mobile SPS receiver, at a given time, based inpart on the transmission received; and transmitting the ordered set ofSPS satellites from a cellular transmission site; such that the mobileSPS receiver may receive the ordered set of SPS satellites.

An apparatus, in one embodiment, facilitating two-way communication witha mobile SPS receiver includes a receiver to receive a transmission,from a mobile SPS receiver, originating within a cell of a cellularservice area, the mobile SPS receiver being configured to transmit andreceive cellular signals; a transmitter to transmit the cellular signalsfrom a cellular transmission site; and a server to determine an orderedset of SPS satellites in view of the mobile SPS receiver; such that theordered set of SPS satellites are transmitted by the transmitter andreceived by the mobile SPS receiver.

The server, in one embodiment, further includes a processor; and aninformation source coupled to the processor. The information sourcecontains sets of SPS satellites in view of cells of the cellular servicearea and the processor determines the ordered set of SPS satellites forthe cell within the cellular service area. The server may be a GPSreference server, a cellular switching center, a location server, acellular transmission site, a base station controller or a mobile SPSreceiver.

A method, in one embodiment, facilitating acquisition of an ordered setof SPS satellites in view of a mobile SPS receiver, via two-waycommunication by a mobile SPS receiver, includes transmitting from acell of a cellular service area to a cellular transmission site thatreceives transmissions from the cell, by a mobile SPS receiverconfigured to receive SPS signals and to transmit and receive cellularsignals. The mobile SPS receiver receives an ordered set of SPSsatellites from the cellular transmission site. The ordered set ofsatellites are those in view of the mobile SPS receiver at a given time;such that the mobile SPS receiver may search for the SPS satellitesaccording to an order of the ordered set of SPS satellites obtained fromthe transmission received from the cellular transmission site. Satellitehealth data may be included in the transmission and the mobile SPSreceiver may modify searching for SPS satellites before or afteracquisition of the SPS satellites based in part on the satellite healthdata. Additionally, the mobile SPS receiver may modify the ordered setsubsequent to receiving.

A apparatus, in one embodiment, facilitating acquisition of an orderedset of SPS satellites in view of a mobile SPS receiver, via two-waycommunication by a mobile SPS receiver, includes a mobile SPS receiverto receive SPS signals; a receiver configured to receive signalstransmitted from a cellular transmission site; and a transmitter totransmit cellular signals to a cellular transmission site; such thatwhen the transmitter, located within a cell of a cellular service area,establishes communication with the cellular transmission site an orderedset of SPS satellites may be transmitted via the cellular transmissionsite to the receiver and the mobile SPS receiver may search for the SPSsatellites according to an order of the ordered set of SPS satellites.

Another embodiment, of the present invention, provides a method toreceive an ordered set of SPS satellites the ordered set beingdetermined by a mobile SPS receiver.

Another embodiment, of the present invention, uses a history of storedGPS satellite signal quality information for a location to determine anordered set of SPS satellites.

Another embodiment, of the present invention, uses mobile SPS receiverinformation to determine an ordered set of SPS satellites.

Another embodiment, of the present invention, includes determining anordered set of SPS satellites in view of a mobile SPS receiver at agiven time; and transmitting the ordered set of SPS satellites to acellular transmission site; such that a server may receive the orderedset of SPS satellites in view of the mobile SPS receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a set of satellites in view of a satellite positioningsystem (SPS) receiver.

FIG. 1B shows a top-down view of the satellites shown in FIG. 1Arelative to the SPS receiver.

FIG. 1C illustrates a cellular communication system having a pluralityof cells each of which is serviced by a cell site, and each of which iscoupled to a cellular switching center.

FIG. 1D illustrates an example of a combined SPS receiver andcommunication system according to one embodiment of the presentinvention.

FIG. 2 shows one embodiment of a cellular based information source whichprovides an association between sets of prioritized orders at giventimes relative to cellular service areas and/or cellular cell sites, inaccordance with the teachings of the present invention.

FIG. 3 is a flowchart illustrating a method for determining aprioritized order of in view satellites in accordance with the teachingsof the present invention.

DETAILED DESCRIPTION

The present invention involves, in one embodiment, the determination ofan ordered set of SPS satellites that are in view of an SPS receiver.The order of the ordered set of satellites is based on the approximatelocation of the SPS receiver, which is determined from identifying orknowledge of a cellular transmission site in cellular communication witha communication system of the SPS receiver. The knowledge of thecellular transmission site may be implicit in the case where the dataidentifying the SPS satellites in view of the SPS receiver is providedby a reference SPS receiver located in geographic proximity to thecellular transmission site communicating with the SPS receiver. In oneembodiment of the present invention, the order of the ordered set ofsatellites is also based on the locations of the satellites relative tothe approximate location of the SPS receiver.

FIG. 1A shows a set of SPS satellites that are in view of an SPSreceiver 100. The SPS receiver also includes a communication system suchas a 2-way cell phone or 2-way (or 1-way) pager. Examples of suchcommunication systems which are coupled to SPS receivers are describedin co-pending U.S. patent application Ser. No. 08/842,559, filed Apr.15, 1997. Also see PCT publication WO 98/25157. FIG. 1B shows a top-downview of the satellites shown in FIG. 1A relative to SPS receiver 100.Satellites 102, 104, 106, 108, 110 and 112 are shown in locations at aparticular time of day. It should be noted that because satelliteschange position over time, some of the SPS satellites shown in FIG. 1Amay not be visible to SPS receiver 100 at different times. Furthermore,SPS receiver 100 is typically mobile. Thus, the satellites that are inview of SPS receiver 100 may change as SPS receiver 100 moves todifferent locations. Further, in other embodiments, the source of SPSsignals may become blocked (e.g., hidden behind a building) or heavilyattenuated. This blocking or attenuation may be taken into account whenselecting an ordered list of satellites, which is described below.

FIG. 1C shows an example of a cellular based communication system 10which includes a plurality of cell sites, each of which is designed toservice a particular geographical region or location. Examples of suchcellular based communication systems are well known in the art. See, forexample, U.S. Pat. No. 5,519,760 which describes a cellular networksystem. The cellular based communication system 10 includes two cells 12and 14 both of which are defined to be within a cellular service area11. In addition, the system 10 includes cells 18 and 20. It will beappreciated that a plurality of other cells with corresponding cellsites and/or cellular service areas may also be included in the system10 and coupled to one or more cellular switching centers, such as thecellular switching center 24 and cellular switching center 24 b.

Within each cell, such as cell 12, there is a wireless cell site orcellular site such as cell site 13 which includes an antenna 13 a whichis designed to communicate through a wireless communication medium witha communication receiver which may be a combined mobile GPS receiver andcommunication system such as the receiver 16 shown in FIG. 1C. Anexample of such a combined system is shown in FIG. 1D and may includeboth a GPS antenna 377 and a communication system antenna 379. It willbe appreciated that alternative embodiments may employ a single antennaor more than two antennas.

Each cell site is coupled to a cellular switching center. In FIG. 1C,cell sites 13, 15, and 19 are coupled to switching center 24 throughconnections 13 b, 15 b and 19 b respectively and cell site 21 is coupledto a different switching center 24 b through connection 21 b. Theseconnections are typically wire line connections between the respectivecell site and the cellular switching centers 24 and 24 b. Each cell siteincludes an antenna and transmitter and receiver for communicating withcommunication systems serviced by the cell site. It will be appreciatedthat a communication system within one cell, such as the receiver 22shown in cell 4, may in fact communicate with the cell site 19 in cell18 due to blockage (or other reasons why cell site 21 cannot communicatewith the receiver 22).

In a typical embodiment of the present invention, the mobile GPSreceiver 16 includes a cellular based communication system, which isintegrated with the GPS receiver, such that both the GPS receiver andthe communication system are enclosed in the same housing. When thiscombined system is used for cellular telephone communications,transmissions occur between the receiver 16 and the cell site 13.Transmissions from the receiver 16 to the cell site 13 are thenpropagated over the connection 13 b to the cellular switching center 24and then to either another cellular telephone in a cell serviced by thecellular switching center 24, or through a connection 30 (typicallywired) to another telephone through the land based telephonesystem/network 28. It will be appreciated that the term wired includesfiber-optic and other non-wireless connections such as copper cabling,etc. Transmissions from the other telephone, which is communicating withthe receiver 16, are conveyed from the cellular switching center 24through the connection 13 b and the cell site 13 back to the receiver 16in the conventional manner.

The remote data processing system 26 (which may be referred to in someembodiments as a GPS server or a location server) is included in thesystem 10 and is used when, in some embodiments, a mobile GPS receiverwithin a particular cell is used to determine the position of thereceiver using GPS signals received by the GPS receiver. The GPS server26 may be coupled to the land based telephone system/network 28 througha connection 27 and it may also be optionally coupled to the cellularswitching center 24 through the connection 25 and also optionallycoupled to center 24 b through the connection 25 b. It will beappreciated that connections 25 and 27 are typically wired connectionsalthough they may be wireless. Also shown as an optional component ofthe system 10 is a query terminal 29 which may consist of anothercomputer system, which is coupled through the network 28 to the GPSserver. This query terminal 29 may send a request for the position of aparticular GPS receiver in one of the cells to the GPS server 26 whichthen initiates a conversation with a particular GPS receiver through thecellular switching center in order to determine the position of the GPSreceiver and report that position back to the query terminal 29.

It should be noted that a cellular based communication system is acommunication system, which has more than one transmitter, each of whichserves a different geographical area, which is predefined at any instantin time. The cell sites may also move, rather than be stationaryterrestial sites; for example, the cell sites in the Iridium and theGlobalstar systems are low earth orbiting satellites. Typically, eachtransmitter is a wireless transmitter, which serves a cell, which has ageographical radius of less than 20 miles, although the area covereddepends on the particular cellular system. There are numerous types ofcellular communication systems, such as cellular telephones, PCS(personal communication system), SMR (specialized mobile radio), one wayand two-way pager systems, RAM, ARDIS, and wireless packet data systems.Typically the predefined different geographical areas are referred to ascells and a plurality of cells are grouped together into a cellularservice area such as the cellular service area 11 shown in FIG. 1C andthese plurality of cells are coupled to one or more cellular switchingcenters which provide connections to land based telephone systems and/ornetworks. Service areas are often used for billing purposes. Hence, itmay be the case that cells in more than one service area are connectedto one switching center. For example, in FIG. 1C, cells 1 and 2 are inservice area 11, and cell 3 is in service area 18, but all three areconnected to switching center 24. Alternatively it is sometimes the casethat cells within one service area are connected to different switchingcenters, especially in dense population areas. In general, a servicearea is defined as a collection of cells within close geographicalproximity to one another. Another class of cellular systems that fitsthe above description is satellite based, wherein the cellularbasestations are satellites that typically orbit the earth. In thesesystems, the cell sectors and service areas move as a function of time.Examples of such systems include Iridium, Globalstar, Orbcomm andOdyssey.

FIG. 1D shows a generalized combined GPS and communication transceiversystem. The system 375 includes a GPS receiver 376 having a GPS antenna377 and a communication transceiver 378 having a communication antenna379. The GPS receiver 376 is coupled to the communication transceiver378 through the connection 380 shown in FIG. 1D. In normal operation,the communication system transceiver 378 receives approximate Dopplerinformation through the antenna 379 and provides this approximateDoppler information over the link 380 to the GPS receiver 376 whichperforms the pseudorange determination by receiving the GPS signals fromthe GPS satellites through the GPS antenna 377. Various embodiments forthe combined system 375 are known in the art and have been described inthe above referenced co-pending applications.

For the locations of satellites 102-112 and SPS receiver 100 shown inFIG. 1A, a prioritized acquisition order of satellites 102-112 isdetermined. This order represents an optimized order for acquiring SPSsignals from the SPS satellites based on, for example, the geometry ofthe satellites relative to the position of SPS receiver 100. In oneembodiment of the present invention, satellites 102-112 are listed inthe order that provides a desirable geometry among satellites 102-112and SPS receiver 100. For example, the elevation and angle of satellites102-112 relative to SPS receiver 100 may be factors in determining theprioritized order. In yet another example, the best prioritizedorder/selection results in the smallest GDOP (Geometric Dilution ofPrecision) and/or PDOP (position dilution of precision) and/or HDOP(Horizontal Dilution of Precision). Typically, a location serverresponsible for message generation chooses the satellites according to a“best-n” method. In one example of such a method, the chosen satellitesare those that best optimize the geometry among their locations and thelocation of the SPS receiver 100. For example, in a best-4configuration, SPS satellites in FIG. 1A would be chosen to fill theordered list of satellites, and they would be ordered as follows(highest priority to lowest priority): 108, 104, 112 and 102. The best-nsatellite selection method provides the information related to satelliteacquisition strategy that should be followed by SPS receiver 100. In oneembodiment of the present invention, SPS receiver 100 may choose to stopthe satellite acquisition process once the best-n satellites have beenacquired. Thus, not all of the in view satellites need to beprioritized. In fact, the less desirable satellites may not be used atall in resolving the location of SPS receiver 100. In another embodimentthe location server may provide assistance for a subset of satellites inview of the SPS receiver, such as a best-n set. The order is typicallychosen to attempt to acquire first those SPS satellites which are nottoo close to the horizon and which provide an optimal geometrictrilateration solution. The former requirement generally means that theSPS signals will be more easily received from satellites not close tothe horizon and the latter requirement means that the position solution(from the pseudoranges to the highest ordered satellites in the order)will have better accuracy (less error) than a position solution whichcould use the lowest ordered satellites in the order. The order mayreflect the expected quality of the measurements (e.g., the firstsatellites in the order are expected to provide higher qualitymeasurements than the remaining satellites in the order).

It should be appreciated that satellite acquisition is a step in thelocation determination process. It is also appreciated that if azimuthand elevation data are provided to an SPS receiver, it could use suchdata to further optimize its satellite acquisition strategy. It isfurther appreciated that criteria other than geometry or relativelocation may be used to prioritize the acquisition order of satellites,such as satellite health.

Satellite health assistance protects against rogue satellitemeasurements. In heavily obstructed signal environments, quite often,GPS satellite signals are received with a very high dynamic range.Receiving GPS signals with signal strengths differing by more thanapproximately 17 dB may cause a GPS receiver to acquire across-correlated signal instead of a relatively weaker true signal. Oneprocedure that may be used to detect and possibly correct or remove across-correlated measurement is described in co-pending U.S. patentapplication Ser. No. 09/241,334, filed Feb. 1, 1999, which is herebyincorporated herein by reference. However, for a GPS receiver to detectthe presence of cross-correlated signals, all signals from both healthyand unhealthy satellites should be acquired. A problem would arise if astrong “unhealthy” satellite signal cross-correlates with a weak“healthy” satellite signal. Unaware of the presence of an “unhealthy”signal, a GPS receiver may not be able to detect a cross-correlationcondition.

In one embodiment of the present invention, GPS reference receiverswhich provide reference data for location servers (also referred to as aPosition Determination Entity (PDE) in CDMA cellular phone systems andServing Mobile Location Centre (SMLC) in the GSM cellular phone systems)acquire and track all satellites in view: healthy and unhealthy.Further, all GPS technologies (e.g. a GPS receiver) integrated with orconnected to wireless devices (e.g. a cellular telephone or a two-waypager) also acquire and track all satellites in view: healthy andunhealthy. In a wireless assisted GPS (WAG) mode (e.g. see examplesdescribed in co-pending U.S. patent application Ser. No. 08/842,559,filed Apr. 15, 1997, this co-pending patent application is herebyincorporated herein by reference), location server(s) may provide“health” status information to the mobiles communicating with a wirelessnetwork served by the location server(s). This health status informationmay accompany any other assistance information provided by the locationserver(s). In general, assistance information allows fast acquisition ofGPS signals in highly restrictive signal environments. In order toachieve such performance improvements, assistance information mayspecify the satellites to be searched for, the estimated time of arrivalof these signals and the expected frequency (Doppler) of the signals.This assistance information may be provided to improve a 3-dimensionalsearch for a satellite signal. When the satellite's signals areacquired, pseudoranges, Dopplers and other satellite signal measurementsare analyzed for cross-correlation conditions. In order to perform thisanalysis, measurements should be made for all satellites in view:healthy and unhealthy. In this embodiment, the satellite healthinformation is used to detect a cross-correlation condition and then thecross-correlated and/or “unhealthy” satellites are analyzed to determinewhether they should be included in the location computation process orcorrected. Where the assistance information is provided for healthysatellites only (satellite health is implied by the satellite list) andthe current and valid satellite health status information is notavailable to the mobile GPS receiver, the mobile will attempt to acquireonly healthy satellites. In this case, a possible presence of a “strong”unhealthy satellite potentially cross-correlating with relatively weakerhealthy satellites would not be known to the mobile and therefore wouldnot be tested for. The use of undetected cross-correlated signals maylead to large positional errors thus affecting the quality of thelocation service.

The prioritized order may be modified based on the satellite healthinformation.

Alternatively, health information may be directly received from thesatellites and this health information may be used in the same mannerdescribed herein as the health information which is received from atransmitter at a cell site.

The health information may be transmitted from a cell site bybroadcasting this information for all satellites in view of a cellulartelephone basestation (“cell site”). Alternatively, it may be providedto a cellular telephone upon request (on demand) for a position of thetelephone; the health information may be transmitted from the cellulartelephone basestation to the cellular telephone which then provides thehealth information to a GPS receiver which is coupled to the cellulartelephone. In the case where the information is transmitted on demand, aGPS server may determine the appropriate (e.g. updated health)information based upon a cell site which is in cellular radio/wirelesscommunication with the phone and this cell site determines anapproximate location which is used to determine satellites in view ofthat location and the updated health information for these satellites isthen caused to be transmitted (in one case) to the cellular telephonewhich in turn provides the information to the mobile GPS receiver foruse in processing SPS signals in the GPS receiver. In another case, theGPS server may retain the updated health information and use it toprocess the pseudoranges (e.g. correlation measurements) received fromthe mobile GPS receiver in order to determine the position of the mobileGPS receiver. In both cases, pseudoranges (e.g. correlation measurementswhich specify code phases) and estimated Doppler are determined even forknown unhealthy GPS satellites so that cross-correlations can bedetected as described here. For example, a GPS receiver may receive theupdated health information from a cell site but still acquire GPSsignals from a GPS satellite which was indicated to be unhealthy in thetransmitted updated health information. Co-pending U.S. application Ser.No. 08/842,559, filed Apr. 15, 1997, describes a method for identifyinga cell site which is in wireless communication with a cellular telephoneand which then determines satellite assistance data for in viewsatellites based upon an approximate location derived from identifyingthis cell site. This method may be used with the present invention wherethe satellite assistance data in this case is either satellite health(e.g. based on satellite almanac) or updated satellite health (e.g. morecurrent than the existing satellite almanac message's informationconcerning satellite health).

In another embodiment, an SPS receiver may, based on stored or acquiredinformation, autonomously determine the optimum order of satellites andprovide the ordered list to a location server which is responsible formessage generation (and which can then provide the ordered list to otherSPS receivers and/or the assistance data in the order as provided by theSPS receiver).

In yet another embodiment, a location server may, based on theinformation provided from a mobile SPS receiver or stored information(e.g., a history of GPS signal quality), determine an ordered list ofsatellites that would reflect the probability of successful signalacquisition (e.g., satellites near the horizon would have a lowerprobability of successful signal acquisition).

FIG. 2 shows an example of a cellular based information source which inone embodiment is maintained at an SPS server such as a globalpositioning system (GPS) server. Alternatively, the information sourcemay be maintained at a cellular switching center, base stationcontroller or at each cell site. Typically, the information source ismaintained and routinely updated at the SPS server which is coupled tothe cellular switching center. The information source may maintain datain various formats, and it is appreciated that the format 200 shown inFIG. 2 illustrates only one example of such formats.

Typically, each set of prioritized order information at a particulartime, such as prioritized order set A1 at time t₁ will include acorresponding location or identification for a cell site or a servicearea. For example, for prioritized order sets A1 and A2 there is acorresponding identification of the cellular service area A as well asthe latitude and longitude for a representative location in this servicearea. It is appreciated that typically this latitude and longitude is an“average” location which is generally centrally located within thegeographical region of the cellular service area. However, otherpossible approximations may be used particularly where the cellularservice area includes terrains which are not used.

As shown in the exemplary cellular based information source of FIG. 2,the cellular based information source includes a column 202 specifyingthe cellular service area, and a column 204 specifying a cellular siteidentification or number. Note that for cellular service area A the cellsite identification or location is not specified, and thus theapproximate location is based on a representative location in thecellular service area, and thus the prioritized orders A1 and A2 foracquiring SPS satellites are based on this location depending on theparticular time, such as times t₁ and t₂. Column 206 includes aspecification of the latitude and longitude for the particularrepresentative location in the service area. Column 208 includes aspecification of the latitude and longitude for the location of aparticular cell site within the cellular service area which may be usedas a representative location for a mobile SPS receiver which receivesthe prioritized order. Column 210 includes the prioritized orders of inview satellites at times t₁ and t₂ for the appropriate representativelocation. In an alternative embodiment, the ordered list (and thecorresponding cellular based information) may be determined inreal-time, near real time, continuously or on-demand.

FIG. 3 is a flowchart illustrating one embodiment of a method fordetermining a prioritized order of in view SPS satellites in accordancewith the teachings of the present invention. In operation 302, anapproximate location of an SPS receiver is determined from a cell basedinformation source. The SPS receiver is in wireless radio/cellularcommunication with at least one wireless cell site, and the identity ofthis cell site is determined. The approximate location is based on atleast one of a representative location in a cellular service area whichincludes this cell or a representative location of the wireless cellsite in the cellular service area and again represents the approximatelocation of the SPS receiver being serviced by the wireless cell site. Acellular based information source (e.g. see FIG. 2) may be used to lookup or determine the approximate location based on the identification ofthe wireless cell site which is in communication with a cellularcommunication system which is coupled to the SPS receiver.Alternatively, the cellular based information source may be used to lookup or determine the appropriate prioritized order directly from theidentification of the wireless cell site which is in communication withthe SPS receiver. In operation 304, a prioritized order based on theapproximate location of the SPS receiver (or the identification of thewireless cell site communicating with the SPS receiver through acellular communication system which is coupled to the SPS receiver) isdetermined for the satellites that are in view of the approximatelocation. In one embodiment of the present invention, the satellites areprioritized according to their location relative to the approximatelocation of the SPS receiver. In another embodiment of the presentinvention, the satellites are prioritized according to their locationrelative to one another and relative to the approximate location. It isappreciated that some criteria other than geometry may be used toprioritize the order. In operation 306, the prioritized order istransmitted from the wireless cell site to the SPS receiver and the SPSreceiver then searches for and acquires SPS signals from the SPSsatellites in the order designated in the prioritized order sent to theSPS receiver.

A more detailed discussion of cellular communication systems and theiruse with SPS receivers is disclosed in U.S. patent application Ser. No.08/842,559, now U.S. Pat. No. 6,208,290, filed on Apr. 15, 1997, titled“An Improved GPS Receiver Utilizing a Communication Link” by Norman F.Krasner.

The prioritized order of SPS satellites provided by the location serveror derived from the SPS receiver may be used to improve time to acquiresatellites, time required to determine location information, and mayreduce the bandwidth requirements for providing data from the locationserver to the SPS receiver.

In this discussion, embodiments of the present invention have beendescribed with reference to application in the United States GlobalPositioning System (GPS) system, which is an example of an SPS system.It should be evident, however, that these methods are equally applicableto other satellite positioning systems, such as the Russian Glonasssystem. Thus, the term “GPS” used herein includes such alternativesatellite positioning systems, including the Russian Glonass system.Likewise, the term “GPS signals” includes signals from alternativesatellite positioning systems.

Furthermore, although embodiments of the present invention are describedwith reference to GPS satellites, it will be appreciated that theteachings are equally applicable to positioning systems which utilizepseudolites or a combination of satellites and pseudolites. Pseudolitesare ground based transmitters which broadcast a PN code (similar to aGPS signal) modulated on an L-band (or other frequency) carrier signal,generally synchronized with GPS time. Each transmitter may be assigned aunique PN code so as to permit identification by a remote receiver.Pseudolites are useful in situations where GPS signals from an orbitingsatellite might be unavailable, such as tunnels, mines, buildings, urbancanyons or other enclosed areas. The term “satellite”, as used herein,is intended to include pseudolites or equivalents of pseudolites, andthe term GPS signals, as used herein, is intended to include GPS-likesignals from pseudolites or equivalents of pseudolites.

In the foregoing detailed description, the apparatus and method of thepresent invention have been described with reference to specificexemplary embodiments. However, it will be evident that variousmodifications and changes may be made without departing from the broaderscope and spirit of the present invention. The present specification andfigures are accordingly to be regarded as illustrative rather thanrestrictive.

1. A method, comprising: determining an ordered set of SPS satellites inview of a location of a cell of a cellular communication system at agiven time, wherein an order of SPS satellites in the ordered set isdetermined in a manner selected from the group consisting of, minimizinga geometric dilution of precision (GDOP), minimizing a position dilutionof precision (PDOP), minimizing a horizontal dilution of precision(HDOP), providing a position solution which uses SPS satellites having adesired geometry relative to one another, providing a position solutionwhich uses SPS satellites having a desired geometry relative to themobile SPS receiver, determining a probability of SPS satellite signalacquisition, determining an estimate of measurement quality from theordered set of SPS satellites, providing an optimal geometrictrilateration solution, and determining a user defined selectioncriteria; and wherein a mobile SPS receiver located within the cell ofthe cellular communication system may receive the ordered set of SPSsatellites, and transmitting the ordered set of SPS satellites.
 2. Amethod, as in claim 1, wherein said determining is done according to aBest-n method and said determining further comprises determiningsatellite health information.
 3. A method, comprising: receiving atransmission from a mobile satellite positioning system (SPS) receiverwithin a cell of a cellular communication system, the mobile SPSreceiver being configured to transmit and receive cellular signal;determining an ordered set of SPS satellites in view of the mobile SPSreceiver, at a given time, based in part on said receiving, wherein anorder of SPS satellites in the ordered set is determined in a mannerselected from the group consisting of, minimizing a geometric dilutionof precision (GDOP), minimizing a position dilution of precision (PDOP),minimizing a horizontal dilution of precision (HDOP), providing aposition solution which uses SPS satellites having a desired geometryrelative to one another, providing a position solution which uses SPSsatellites having a desired geometry relative to the mobile SPSreceiver, determining a probability of SPS satellite signal acquisition,determining an estimate of measurement quality from the ordered set ofSPS satellites, providing an optimal geometric trilateration solution,and determining a user defined selection criteria, and transmitting theordered set of SPS satellites; such that the mobile SPS receiver mayreceive the ordered set of SPS satellites.
 4. A method, as in claim 3,wherein said determining is done according to a Best-n method and saiddetermining further comprises determining satellite health information.5. A computer readable medium containing executable computer programinstructions which when executed by a data processing system, cause thedata processing system to perform a method comprising: determining anordered set of satellite positioning system (SPS) satellites in view ofa location of a cell of a cellular communication system at a given time,wherein an order of SPS satellites in the ordered set is determined in amanner selected from the group consisting of, minimizing a geometricdilution of precision (GDOP), minimizing a position dilution ofprecision (PDOP), minimizing a horizontal dilution of precision (HDOP),providing a position solution which uses SPS satellites having a desiredgeometry relative to one another, providing a position solution whichuses SPS satellites having a desired geometry relative to the mobile SPSreceiver, determining a probability of SPS satellite signal acquisition,determining an estimate of measurement quality from the ordered set ofSPS satellites, providing an optimal geometric trilateration solution,and determining a user defined selection criteria; and transmitting theordered set of SPS satellites.
 6. A computer readable medium, as inclaim 5, wherein said determining is done according to a Best-n methodand said determining further comprises determining satellite healthinformation.